An optical network, such as a passive optical network (PON), includes a number of optical line terminals (OLTs) that are connected to a number of optical network units (ONUs) using optical fibers. For example, an OLT in a centralized location may be connected to multiple ONUs that are in different locations.
The optical fiber connected to an OLT is split into multiple tributary branches at an optical splitter, and each tributary branch is connected to a respective ONU. Optical signals (light) from the OLT are carried through the main branch of the optical fiber to the splitter, then through the tributary branches to the ONUs. Optical signals (light) from each ONU are carried through the respective tributary branch to the splitter, and then through the main branch to the OLT. Thus, streams of optical signals from the OLT to the ONUs, and from multiple ONUs to the OLT, use the same main branch of the optical fiber. A protocol such as time division multiple access (TDMA) can be used to distinguish one stream from another.
Single-mode optical fiber is usually used in a practical PON for its high bandwidth. In a single-mode optical fiber, only the fundamental mode can propagate for a long distance; other higher modes will leak and vanish quickly. In a typical implementation, a single-mode optical fiber is split into 64 single-mode tributary branches. Ideally, in such an implementation, 1/64 of the light ( 1/64 of the optical signal power) transmitted by an OLT would reach each ONU. In actuality, only about 1/100 of the optical signal power reaches each ONU because of signal attenuation along the transmission path, due to factors such as absorption, scattering, bends in the optical fiber, losses due to leaky modes, and losses at connectors and splices, for example. The direction from the OLT to the ONUs is usually referred to as the downstream direction.
Light coming in the other direction—the upstream direction, from the ONUs to the OLT—also suffers from similar power losses along the transmission path. Due to intrinsic features of waveguide coupling, most of the light in the fundamental mode in the single-mode tributary branches enters higher-order modes in the main branch and leaks quickly in the main branch. As a result, only about 1/100 of the optical signal power transmitted by an ONU reaches the OLT, resulting in very high insertion losses in the upstream direction.
Losses in the downstream direction are often acceptable, because most of the optical signal power is distributed to different ONUs and is utilized. What is more, downstream losses can be compensated for by placing an optical amplifier in the main branch, for example. However, losses in the upstream direction are problematic because most of the optical signal power just leaks and is wasted, and it is too expensive to place an optical amplifier in each tributary branch. Upstream losses can reduce bandwidth and reduce the transmission distance of the optical signals.
Accordingly, a solution that can reduce the upstream insertion losses at the splitter would be beneficial.